1. Field of the Invention
This invention relates to a method and apparatus for heating and sterilizing food and, more particularly, to a food heating and sterilizing method and apparatus adapted to heat and sterilize food which is in open or unsealed condition.
2. Prior Art
Conventionally, solid food is generally heated and sterilized under pressure after the food is filled in sealed containers. However, with such a conventional method of sterilization, the food can be susceptible to bear an odor specific to retort treatment because it is mostly contained in such sealed containers and/or requires a particularly long processing time because the food has to be heated with air contained in sealed containers.
A number of different methods have been proposed to solve this problem. For example, Japanese Patent Laid-Open Publication No. 4-370081 discloses a method of filling boiled rice into containers, heating and sterilizing it under pressure and thereafter sealing the containers under an aseptic condition.
On the other hand, a variety of sterilizing apparatuses are already known for successively feeding food into a sealed sterilizing chamber for heating and sterilizing the food at high temperature or at high temperature and under high pressure, including those disclosed in Japanese Patent Laid-Open Publication Nos. 49-71177, 51-42391 and 64-51069. With any of these sterilizing apparatuses, however, a conveyor means has to be arranged within the food heating and sterilizing chamber of the apparatus in order to advance the food fed to the apparatus further into the chamber. A conveyor means arranged within a sealed chamber can make the entire configuration of the apparatus rather complicated and disadvantageous from the viewpoint of maintaining the chamber in an aseptic state.
With the method disclosed in Japanese Patent Laid-Open Publication No. 4-370081, the food is simply sterilized in a pressurized steamy atmosphere and, when solid food such as a lump of boiled rice containing a large number of small void spaces therein is to be sterilized, air remaining in those spaces can prevent any quick and efficient sterilization from taking place.
Additionally, air remaining in small void spaces can interfere with uniform temperature rise (distribution) in the food. Thus, if excessive heat is applied to the food to raise the temperature of the center of the food to a sufficiently high level for sterilization, the surface of the food can become discolored and the food itself can be degraded through the excessive heating.
On the other hand, a variety of food heating and sterilizing apparatuses have been proposed to successively move packages of food into a sealed and heated sterilizing chamber without arranging a conveyor means in the chamber in view of the problems that may arise in known food heating and sterilizing apparatuses comprising a sterilizing chamber in which a conveyor means is installed. For example, U.S. Patent Application Ser. No. 08/395,111 filed on Feb. 27, 1995 and assigned to the assignee of the present application, discloses a food heating and sterilizing apparatus comprising a heating vessel (a heater) for sterilizing packages of food fed successively thereto at high temperature and under high pressure. In the disclosed apparatus, a heating chamber filled with hot and pressurized steam and a cooling chamber held to a temperature lower than that of the heating chamber are connected with each other by way of a sealing gate, which is opened at regular intervals to allow the food sterilized in the heating chamber to be advanced into the cooling chamber. With this arrangement, low temperature air can flow from the cooling chamber into the heating chamber whenever the sealing gate is opened to interfere with the conduction of heat to the food and consequently lower the efficiency with which the food is heated.
Additionally, steam in the heating chamber can be condensed to water to accumulate in the heating chamber due to the cold air flow from the cooling chamber. The water to be drained can boil up abruptly or bump (reboiling phenomenon occurs) when the sealing gate is opened and the inner pressure of the heating chamber is lowered, so that the food in the chamber is wetted and degraded.